Process for improvement of malleable iron castings



Patented Dec. 10, 1935 PROCESS FOR. IMPROVEMENT OF MALLE- ABLE IRON CASTINGS Cyril Stanley Smith, Cheshire, Conn., assignor, by mesne assignments, to Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio No Drawing. Application October 13,1932, Serial No. 637,671

8 Claims.

This invention relates to the production of malleable iron castings of superior strength, the improvement resulting from the addition of a substantial amount of copper combined with a process of heat treatment, following the usual malleabilizing anneal, which has for its object the dispersion of the copper in a suitable submicroscopic form to cause the so-called precipitation hardening.

In the production of malleableiron castings the customary method consists of making white:

fracture iron castings from a molten metal containing 2 to 3 percent carbon, 0.8 to 2 percent silicon, with small amounts of manganese and traces l5 s of sulphur and phosphorus. The composition is adjusted so that the castingscontain substantially all the carbon in the combined form and are very hard and brittle. The castings are then annealed for prolonged periods at temperatures customarily in the neighborhood of 850 to 950 C. to cause the undissolved combined carbon or iron carbide to change to graphite and cooled slowly to a temperature below the critical point. Various modifications of this process resulting in a shorter time have been proposed, but all have for their object the production of a final microstructure consisting of rounded particles of temper carbon embedded in a soft iron matrix with practically no combined carbon. Metal having this structure possesses considerable ductility.

Initsbroadest aspects the invention comprises the precipitation hardening of malleable cast iron by the addition of any element whose solubility in alpha iron or iron below the critical point increases with an. increase in temperature. The

hardening results from a heat treatment comprising heating to a temperature where a substantially greater amount of the added element is dissolved than is soluble at room temperature, cooling at a rate sumcient to retain the excess of the added element in solution, and reheating to a temperature at which precipitation of the element will occur. Under suitable conditions of dispersion the precipitation will result in consid- 'erable hardening. I have found copper to be a very suitable element to use in securing the precipitation hardening.

I have found that copper up to about 5 percent may be incorporated with the molten metal before casting without harmful eifect on the process of malleabilizing or graphitizing, but the finally -malleabilized casting can then be greatly improved byv giving a so-called precipitation hardening treatment, that is, a heat treatment to dissolve the copper and, according to the present (Cl. 148-21a8) accepted theory, reprecipitate it in a sub-microscopic form of such a nature that the soft iron matrix is considerably strengthened without very much loss of ductility. In its essentials, the heat treatment consists of heating the copper-contain- 5 ing malleable casting to a suitable temperature for sufficient time to dissolve a large amount of the copper (a heat treatment hereinafter referred to as the solution heat treatment) cooling at a rate sufliciently great to retain most of the cop- 10 per in solution and then reheating to an intermediate temperature for suflicient time to enable the excess copper to be'thrown out of solution in a critically dispersed condition, to produce substantially maximum hardening. 15

The molten iron used for the production of castings under this invention may be of any composition which will give an annealable white iron casting, employing either sand or permanent molds, and is distinguished from that used in 20 customary practice by the addition of copper in amounts sufiicient to respond to the precipitation hardening treatment. k

' The smallest amount of copper which will result in precipitation hardening is about 0.6 per- 25 cent and, while the maximum effect occurs at 1 to 1.5 percent copper, for some purposes it may be desirable to have as much as 5 percent copper, although for most purposes it probably would not be desirable to go above about 3 percent. The 3 white iron castings containing copper may be rendered malleable by any of the customary graphitizing processes; either the slow treatment in which the castings, usually packed in boxes and supported in scale, sand or other oxidizing 35 or non-oxidizing packing material, are subjected to a cycle of heating to a temperature above the critical temperature (A1) andvery slow cooling to a temperature below the critical temperature, or any of the recently developed short cycle proc- 40 esses which by a suitable control of the temperature complete graphitization much more rapidly. In any case, save for the somewhat shorter time required when copper is present, the treatment up to this point is indistinguishable from the 45 customary methods and the product is a malleable casting distinguished only by a slightly greater strength and better corrosion resistance on account of its copper content. Microscopically the structure of the malleable iron in this stage con- 50 sists of ferrite, temper carbon (graphite), and copper which is distributed in the form of microscopically visible particles scattered throughout the ferrite in amounts depending upon the excess of the copper content above the solid solu- 55 position.

bility of copper in ferrite at the lowest temperature of the malleabilizing process There may also be small quantities of pearlite or cementite if the malleabilization. was accidentally or intentionally stopped before graphitization was complete. The presence of the copper particles is all that distinguishes the microstructure of the metal from ordinary malleable iron in this stage.

In my invention the complete malleabilized iron containing copper is then given a final heat treatment to develop precipitation hardening. The malleabilized castings are heated to a temperature at which a substantial portion of the copper will be redissolved in solid solution, which temperature may preferably be in the range from 700 to 850 C. (1290 to 1565 F.) and then quenched, air cooled or cooled byother means but at a rate faster than approximately 25 C. (45 F.) per hour'toa temperature belowapproximately 600 C. and; which may be. to approximately room temperature, and reheated or maintained for a suitable length of time at a temperature in the range of 400 to 600 C. (750 to 1110' F.) followed by final cooling at any speed tically identical composition save for the presence of one percent copper in melt B.

Change in properties of malleable iron due to heat treatment MdtlA" MdtllBl) 0.0 percent copper 1.0 percent coppc' Temperature of solution heat I tmen Increase Increase Inueese Increase in yield in tensile in in tensile point strength point strength [tale in Ilia/sq. in. [ta/lg. h. lba/eg. in.

2, 450 1, 500 l, 150 -l, 700 4, 750 4, 6m 2 550 2 ll!) 3, 050 3, 300 850 4(1) 5, 650 -5. 100 2 850 5(1) 3, 150 3, 800 450 400 14. 850 8, 500 2, W l, 700 l, 750 l, 700 550 ill) 11, 350 10, too 1, 550 2, 8X) 3, 350 8, 900 750 1, ill) 15. 950 20, 700 11, 560 21, 900 12 650 20, 300 12,050v 24,;100 28,350 40, 100 25, 950 37, 400 31 850 44, 900 28, 550 40, 42, 350 56, 500

cetes sample reheated 4 hours at The temperature of the solution heat treatment must be high enough to dissolve a substantial amount of copper, at least above 730 C.- in the iron considered above, but this will vary with com- Greater ductility will result if the temperature is below the critical point (about 760 C. in most malleable iron, somewhat less when copper is present) and if this is exceeded the resulting re-solution of carbon will result in a further increase of strength but with loss of ductility. It is an advantage to use a reasonably high temperature to obtain sufficient copper in solution and a small amount of carbon re-solution resulting in the final production of pearlitic or sorbitic patches scattered throughout the structure is not harmful andrfor some uses may be definitely advantageous.- 5

Microscopically, the precipitation hardening described does not change the structure other than by causing the disappearance of the fine copper particles existing in the malleabilized material. The cooling is sufiiciently rapid to pre- 10 vent visible precipitation of copper and the final precipitation heat treatment at about 500 0. causes a change which is quite invisible under the microscope. Treatment above the critical point dissolves some carbon which diffuses and 16 gives rise to pearlitic patches on cooling, but the main purpose of the invention is to cause precipitation of the copper, regardless of pearl-' ite formation.

It will be obvious to one skilled in the art that 20 a number of modifications of the heat treating process described are possible, and it should be understood that this invention is not restrictedto the simple air coolingand reheating treatw ment described above, but any modification of 25 this procedure that has for its aim the precipitation hardening of the metal by suitable dispersion 'of the copper is covered by the present invention. For instance, it is unnecessary to cool the malleabilized castings to room temperature before heat treatment, but the castings-either in a separate furnace or in e one used for the malleabilizing annealma be heated directly from the final malleabilizing temperature to the proper temperature for putting the copper into solution, and the process thereafter continued as described by cooling and reheating. A sample malleabilized and then directly heated from the. final malleabilizing temperature to 745 0., maintalned for one hour and air cooled had a Brineil 40 hardness number of 120. On reheating the sample for four hours at 500 C. to cause precipitation the hardness increased to 138, showing that the solution heat treatment may follow the malleabilizing treatment directly without intermedi- 45 ate cooling and without appreciable eflect on the properties. I have also found that itis unnecessary to cool to room temperature from the solution heat treatment, whether this follows directly the malleabilizing or follows cooling to room tem- 50 perature, but the cooling may be stopped at the desired temperature and the precipitation treatment continued directly thus saving both heat and time. As a specific example, a sample of cast iron containing one percent copper was malleabilized by a suitable treatment and the temperature raised directly without lntermedi ate cooling to 745 C., held one hour to permit the copper to dissolve, withdrawn from the furnace and cooled in the air to aboutv 500 0., when 00 it was placed in a second furnace maintained at about 500 (7., held at this temperature for four hours and finally cooled to room' temperature. This sample, which had not been cold from th time of charging in the malleabilizing furnace l6 to-the end of the precipitation treatment, had a Brinnell hardness of 142, equal to that obtained by an intermittent process.

The proper duration of the precipitation treat ment depends on the temperature and if the proper combination of time and temperature is exceeded the metal softens again. At 450 0. about 24 hours, at 500 C. about 4 hours, and at 550 C. about 2 hours suflice, while at 600 0.

about 15 minutes give maximum hardness, and It I properties are somewhat better, but commercial operations may well require the shorter time of treatment resulting from the use of higher temperatures.

It will sometimes prove advisable to quench the specimens rather than to air cool them following the solution heat treatment, but in general I prefer to air cool since this is sufllciently rapid to retain most of the copper in solution and it avoids the danger of warping which exists when castings of complicated sections are quenched. The castings may also be cooled from the solution heat treatment in a furnace or by any other means provided that the rate of cooling above about 500 C. is not less than approximately C. per hour. When it is desired to give the precipitation treatment directly without cooling to room temperatures, a molten bath of fusible salt or metal maintained at the desired precipitation temperature may be used to obtain rapid cooling.

Having thus set forth the nature of my invention, what I claim is:

1. The process for the heat treatment of malleable iron castings containing 0.6 to 5 percent copper comprising heating the malleabilized castings to a temperature in the range of approximately 700 to 850 C.; cooling at a rate'greater than. approximately 25 C. per hour to a temperature in the range of approximately 400 to 600 C.; and without further cooling maintaining in that temperature range for suflicient time to produce a substantial increase in hardness.

2. The process for the production of malleable iron castings comprising making castings from a white iron containing 0.6 to 5 percent'copper;

annealing the castings to convert substantially all the carbon into the uncombined state; raising the temperature into the range of approximately 700 to 850 C.; cooling at a rate greater than approximately 25 C. per hour to a temperature in the range of approximately 400 to 600 C.; and without further cooling maintaining in that temperature range for sumcient time to produce a substantial increase in hardness.

3. The process for the heat treatment of white iron castings containing 1.75 to 3.0 percent carbon, 0.6 to 2.5 percent silicon, 0.6 to 5 percent copper, the balance principally iron, comprising heating the castings to a temperature above the carbon critical point, A1, for a suilicient time to convert substantially all the undissolved combined .carbon to the form of temper carbon; cooling to atemperature below A1 and holding for a time suificient to convert substantially all of the remaining carbon to the form of temper carbon; without further cooling heating to a temperature in the range of 700 to 850 C.;

cooling to a temperature of 400 to 600 c. and

without further cooling maintaining in this temperature range for a time suflicient to produce an increase in hardness and tensile strength.

- 4. The process for the heat treatment of white 5 iron castings containing 2.0 1702.5 percent carbon, 0.8 to 1.2 percent silicon and about 1 percent copper, comprising annealing the castings to convert substantially all of the carbon to the form of temper carbon; reheating to a temperature of from about 745 C. to about 800 C.; cooling faster than about 25 C. per hour to a temperature in the neighborhood of 500 C. and without further cooling maintaining at this temperature until an appreciable increase in hard- 5 ness has occurred.

5. The process for the production of malleable iron castings comprising incorporating from about 0.6 to about 5 percent copper with the molten iron before pouring, malleabilizing the 20 castings, heating to a temperature range of 700 to 850 C., cooling at a rate faster than about 25 C. per hour to a temperature of 400 to 600 C., and without further cooling maintaining at a temperature of 400 to 600 0. for sufficient 25 time to produce an increase in hardness and tensile strength.

' 6. The process for the production of malleable iron castings comprising making castings from white iron containing from about 0.6 to about 5 percent copper, annealing the castings to convert substantially all of the carbon into the uncombined state, heating to a temperature in the range of 700 to 850 C., cooling faster than a rate of about 25 C. per hour to a temperature of 400 to 600 C., and without further-cooling maintaining at a temperature of 400 to 600 C. for a suilicient time to produce an increase in hardness and strength.

7. The process for the production of malleable 40 iron castings comprising making white iron castings containing approximately 1 percent copper, annealing the castings to convert substantially all of the carbon to' the form of temper carbon, heating to a temperature of about 745 0., cooling faster than about 25 C. per hour to a temperature in the neighborhood of 500 C., and without further cooling maintaining at a temperature in the neighborhood of 500 C. for about four hours.

8. The process for the heat treatment of malleable iron castings containing approximately 1% to 1.5% copper, comprising heating the malleabilized castings to a temperature in the range of 700 to 850 0., cooling at a rate greater than approximately 25 C. per hour to an approximate range of 400 to 600 C., and with- I out. further cooling maintaining in that tempera- 

